The long-term goal of our laboratory is to understand the mechanisms that control the growth, differentiation, and regeneration of the liver. Among the most profound consequences of disordered hepatocellular growth and differentiation is the development of hepatocellular carcinoma (HCC). HCC is the fifth most common cancer and the third leading cause of cancer deaths worldwide, resulting in over 1 million deaths per year. Its incidence is rising, particularly in the United States, where it has increased by 70% over the last 25 years. No effective secondary prevention or systemic treatments are available. We have a longstanding interest in the role of epidermal growth factor (EGF and its homologs), and the ErbB tyrosine kinase receptors (including the EGF receptor, EGFr) in the control of hepatocellular growth and differentiation. Gefitinib, a small molecule inhibitor of the EGFr tyrosine kinase, will reduce the development of HCC in rats exposed to N- nitrosodiethylamine (DEN), which induces liver injury, cirrhosis, and ultimately HCC. Gefitinib and other EGFr tyrosine kinase inhibitors have recently shown promise in the treatment of human HCC, indicating an important role for the ErbB proteins in hepatocellular carcinogenesis and the relevance of animal models for human HCC. We have shown that only three ErbB proteins are expressed in liver under normal circumstances (EGFr, ErbB2 and ErbB3), and that there is a developmentally-regulated pattern in their expression by hepatocytes. Although they are frequently studied in isolation, the ErbB proteins are highly interactive and form signaling homo- and heterodimers. In human HCC, ErbB3 is one of 217 proteins (out of 9000 examined) that is consistently upregulated;either EGFr or ErbB2 (but not both) tend to be upregulated along with ErbB3. The hypothesis under investigation is that ErbB tyrosine kinases are critical to development of HCC in the DEN rodent model. We propose a systematic study of the consequences of genetic deletion of these molecules in hepatocytes in order to assess their importance in the initiation and progression of HCC. We have generated hepatocyte-specific single and double gene ErbB knockout mice. These animals develop normally and are ideal genetic models to analyze the progression from hepatitis to cirrhosis to hepatocellular carcinoma (HCC) in the setting of ErbB loss. We also have access to a mouse bearing a hypermorphic allele of the EGFr, dsk5, that signals more robustly in response to ligand activation. We expect to identify the specific ErbB receptors and downstream pathways involved in liver carcinogenesis. These studies should assist in the development of molecular targets for interventions to prevent or treat HCC in humans. PUBLIC HEALTH RELEVANCE: Hepatocellular carcinoma (HCC), or liver cell cancer, is the fifth most common cancer, and is the third leading cause of cancer death;the incidence of HCC has risen by over 70% over the past 25 years in the United States. There is significant evidence that a family of proteins called the ErbB tyrosine kinases play a role in the development of liver cancer;they would be extremely good targets for the control of HCC. Using genetically altered mice in which the hepatic ErbB proteins have been rendered nonfunctional or hyperfunctional, we expect to identify key targets for the development of novel chemotherapy to treat HCC.